It seems the act of crossing one’s fingers of which I am most familiar is a sign of good luck. For example, we always crossed our fingers for good luck as a kid. Only on sitcoms and cartoons did I become familiar with a twist to this practice where crossing one’s fingers while making a promise exempts the person from the obligation. Where do these two customs come from (at least, these are the customs I know of within the USA)? Did Hollywood introduce the latter? Last, does the act of crossing one’s fingers have a Christian undertone, like forming a cross with your fingers by the act of crossing them? What’s the factual SD?
It seems that it comes from a time when being a christian was something secret and the crossed fingers were a sort of secret sign like a Masonic handshake.
I knew this custom as a kid in the late 50s early 60s. I don’t think it came from a TV or cartoons. The usual sequence was" You promised!" “Nut uh I had my fingers crossed.” and you’d pull your hand from behind your back with your fingers crossed. (One did not do this with parents or teachers of course.)